SQL Clone
SQLServerCentral is supported by Redgate
 
Log in  ::  Register  ::  Not logged in
 
 
 


Variables, Nulls and Coalesce


Variables, Nulls and Coalesce

Author
Message
Tom Garth
Tom Garth
UDP Broadcaster
UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)UDP Broadcaster (1.5K reputation)

Group: General Forum Members
Points: 1469 Visits: 1499
Ya got me.

Crying

Tom Garth
Vertical Solutions

"There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -- Will Rogers

dun
dun
Mr or Mrs. 500
Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)Mr or Mrs. 500 (564 reputation)

Group: General Forum Members
Points: 564 Visits: 254
This was a great instructional puzzle. I appreciate the corporate knowledge this forum represents. Thanks.
Kevin Gill
Kevin Gill
Ten Centuries
Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)

Group: General Forum Members
Points: 1229 Visits: 356
dun (11/26/2009)
This was a great instructional puzzle. I appreciate the corporate knowledge this forum represents. Thanks.

Glad you enjoyed it :-) This example actually came to mind because of the amount of times I fell over things like this when I was doing serious T-SQL programming for the first time a couple of years ago, when trying to improve and modify the thousands of lines of code left for us to ... try to deal with ... by an (SEI Level 5 - ha!) outsourcing company who wrote our then brand-new system.

-- Kev

-------------------------------
Oh no!
Rob Goddard
Rob Goddard
Right there with Babe
Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)Right there with Babe (766 reputation)

Group: General Forum Members
Points: 766 Visits: 558
Nice question.

Along the same lines of "no rows returned, do nothing", I have a feeling that it may be a case of "400 rows returned, assign 400 values to the same variable" so you are just left with the last one it assigned.

My question is, if you're not doing anything with that variable (i.e. "select @val = col from tab" rather than "select @val = @val + col from tab"), does the optimizer spot this and skip all the variable assignments and just do the last one, or are you inadvertantly creating overhead by assigning every single value that you will never see?

--------
I love deadlines. I like the whooshing sound they make as they fly by. -Douglas Adams
Carlo Romagnano
Carlo Romagnano
SSCertifiable
SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)SSCertifiable (5.9K reputation)

Group: General Forum Members
Points: 5942 Visits: 3344
Rob Goddard (11/27/2009)

does the optimizer spot this and skip all the variable assignments and just do the last one?

All the variable assignments are done.

I run on tuttopodismo
Kevin Gill
Kevin Gill
Ten Centuries
Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)Ten Centuries (1.2K reputation)

Group: General Forum Members
Points: 1229 Visits: 356
Carlo Romagnano (11/27/2009)
Rob Goddard (11/27/2009)

does the optimizer spot this and skip all the variable assignments and just do the last one?

All the variable assignments are done.

I spose this makes sense given the quick method of building a csv from a column i.e. it's clearly possibly that a single variable declaration from a select which returns multiple rows can make use of information from every row.
For the optimiser to spot that it was being assigned to a single variable AND that it would only require data from one row to create the necessary value, it would have to be an exceedingly clever piece of logic.

-- Kev

-------------------------------
Oh no!
Ashka Modi
Ashka Modi
SSC Veteran
SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)SSC Veteran (263 reputation)

Group: General Forum Members
Points: 263 Visits: 112
nice one! Smile its strange!

Thanks,

Ashka Modi
Software Engineer || credEcard Technologies (india) Pvt. Ltd.
Bhavesh_Patel
Bhavesh_Patel
Right there with Babe
Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)Right there with Babe (737 reputation)

Group: General Forum Members
Points: 737 Visits: 297
He I got this one ... :-P



Bhavesh Patel

http://bhaveshgpatel.wordpress.com/
Tom Thomson
Tom Thomson
SSCoach
SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)

Group: General Forum Members
Points: 17796 Visits: 12338
It would have been a good idea to reference http://msdn.microsoft.com/en-us/library/ms187330.aspx as well as the coalesce page, because people thinking that if no value is returned the value is set to null is a pretty common error. I must have explained a few dozen times over the years that

select @a = iValue from @vals where iKey <> iValue

and

set @a = (select  iValue from @vals where iKey <> iValue)

behave completely differently when the filter passes no rows.

Tom

kapil_kk
kapil_kk
Hall of Fame
Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)Hall of Fame (3.7K reputation)

Group: General Forum Members
Points: 3662 Visits: 2766
good tricky question :-)

_______________________________________________________________
To get quick answer follow this link:
http://www.sqlservercentral.com/articles/Best+Practices/61537/
Go


Permissions

You can't post new topics.
You can't post topic replies.
You can't post new polls.
You can't post replies to polls.
You can't edit your own topics.
You can't delete your own topics.
You can't edit other topics.
You can't delete other topics.
You can't edit your own posts.
You can't edit other posts.
You can't delete your own posts.
You can't delete other posts.
You can't post events.
You can't edit your own events.
You can't edit other events.
You can't delete your own events.
You can't delete other events.
You can't send private messages.
You can't send emails.
You can read topics.
You can't vote in polls.
You can't upload attachments.
You can download attachments.
You can't post HTML code.
You can't edit HTML code.
You can't post IFCode.
You can't post JavaScript.
You can post emoticons.
You can't post or upload images.

Select a forum

































































































































































SQLServerCentral


Search